Radio frequency receivers have traditionally used filters such as Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) filters. However, in order to reduce the component size and power consumption of filters used in such radio receivers, and to make such filters more compatible with integrated processing techniques such as CMOS fabrication, frequency translation filter techniques have been developed.
FIG. 1 shows the basic structure of a frequency translation filter. A passive mixer 101 comprising switching elements 101a to 101d is used to frequency translate a frequency dependent impedance in the form of a capacitor 103. The operation of the mixer 101 may be viewed as alternating the polarity of the capacitance 103 using a local oscillator (LO) signal to control the switching elements 101a to 101d, the LO signal having a frequency fLO and period TLO.
In practice two frequency translation filters are required in a radio receiver, which operate with an in-phase (LOi) and a quadrature-phase (LOq) local oscillator signal respectively. Referring to FIG. 2, a first frequency translation filter comprises a passive mixer 201 comprising switching elements 201a to 201d which are used to frequency translate a frequency dependent impedance in the form of a capacitor 203. A second frequency translation filter comprises a passive mixer 205 comprising switching elements 205a to 205d which are used to frequency translate a frequency dependent impedance in the form of a capacitor 207. The LO signal has a 25% duty cycle such that only one of the frequency translation filters 201, 205 is active at a time.
As shown in FIGS. 3a and 3b, respectively, a frequency translation filter (FTF) may be used simply as a load to an amplifier 301 (i.e. a frequency dependent impedance Zin for the amplifier 301 in FIG. 3a), or as a combined mixer and filter with a baseband output Vout (the combined unit 303 shown in FIG. 3b).
The frequency translation filter technique has been proven to be successful for improving the selectivity of a RF front-end in a radio receiver, by effectively giving a band-pass characteristic of the RF circuit.
Recent development of radio transmission schemes suggests the use of multiple simultaneous carriers, referred to as carrier aggregation in the 3GPP Long Term Evolution (LTE) standard. In the future these carriers do not necessarily have to be contiguous in frequency. Simultaneous transmission over several non-contiguous carriers has also been proposed as a possible scenario for white-space and cognitive radio application.
One method developed by the present applicant for handling multiple non-contiguous carriers at a radio receiver is to use complex IF based receivers where a single RF front-end is used to receive multiple desired carriers (as well as undesired signals) and where the desired signals are obtained by means of complex IF mixing or a combination of zero-IF and complex-IF techniques. FIG. 4 shows an example of one such complex configuration, whereby the RF LO frequency is set to a frequency in the middle of two carriers of interest and the IF LO frequency is set to half the distance between the centre frequencies of the two carriers. A complex mixer 401 is used in such an arrangement for handling the multiple carriers. The technique shown in FIG. 4 targets scenarios where the carriers of interest are close to each other compared to their centre frequencies.
As such, the circuit arrangement of FIG. 4 is complex, and a costly method of handling multiple received carriers with non-contiguous frequencies.